In recent years, a filter for trapping diesel particulates is provided in an exhaust pipe of a diesel engine mounted in a vehicle, in order to reduce diesel particulates contained in the exhaust gas.
The filter is made of a porous ceramic body having multiple exhaust gas passages, so that the diesel particulates are trapped by the filter when the exhaust gas passes through porous partitioning walls, which define the multiple exhaust gas passages. A pressure loss is increased, as the trapped (accumulated) amount of the diesel particulates is increased. Then, an engine efficiency is decreased. Accordingly, the filter is re-generated by burning the trapped diesel particulates when the accumulated amount of the diesel particulates becomes larger than a predetermined amount.
According to Japanese Patent Publication No. 2004-286019, an accumulation characteristic is prepared, in which a relation between the accumulated amount of the diesel particulates and the pressure loss is defined. Then, the accumulated amount of the diesel particulates is presumed based on the pressure loss, and the filter is re-generated when the presumed accumulated amount of the diesel particulates exceeds a predetermined value.
According to a further investigation of the inventors, for example, in the apparatus of the above Japanese Patent Publication, the accumulated amount and/or density of the diesel particulates trapped in the partitioning walls will be varied depending on flow velocity of the exhaust gas passing through the partitioning walls. As a result, the accumulation characteristic for defining the relation between the accumulated amount of the diesel particulates and the pressure loss is changed. Namely, the presuming accuracy for the accumulated amount of the diesel particulates becomes worse.